Strong interference has become a common problem as the radio spectrum has become more crowded. Constant envelope, or approximately constant envelope signals are popular because such waveforms are compatible with non-linear amplifiers which can be more energy efficient than linear amplifiers. Examples of constant envelope signals include: frequency modulation, frequency shift keying, minimum shift keying, Gaussian minimum shift keying, multi-h continuous phase frequency modulation, linear FM, continuous wave, and many frequency hopping signals. Any of these types of constant envelope signals can cause interference with other, desired signals, particularly where the desired signal and the constant envelope signal spectrally overlap with one another. Non-constant envelope interference also can be problematic, both with constant envelope signals and non-constant envelope signals with which the interference overlaps. Interference from sources other than the desired source can be intentional or inadvertent.
Thus, what is needed are improved circuits and methods for reducing interference that spectrally overlaps a desired signal.